The present invention relates to a solid state lithium cell having a good discharge characteristic, and more particularly to an improvement of the cathode-active material for such a cell.
Reflecting a rapid progress of IC and LSI in the field of electronics in recent years, there is an increasing demand for highly dependable cells having a long useful life as a power source for such electronic devices which require a minimal electric current.
Under these circumstances, attention is drawn to a solid state cell having a construction in which a solid electrolyte having good ionic conductivity is sandwiched between a cathode-active material and an anode-active material, and which is highly dependable without a problem of liquid leakage.
As solid state cells of this type, there have already been known one wherein lead iodide (PbI.sub.2) is used as the cathode-active material, lithium nitride (Li.sub.3 N) as the solid electrolyte and lithium metal (Li) as the anode-active material, or one wherein a mixture of PbI.sub.2 and lead sulfide is used as the cathode-active material, a mixture of lithium iodide (LiI) or water-containing crystals of LiI (LiI.multidot.H.sub.2 O, LiI.multidot.2H.sub.2 O or LiI.multidot.3H.sub.2 O) and .alpha.-alumina or silicic acid anhydride (SiO.sub.2) as the solid electrolyte, and metal lithium as the anode-active material. (U.S. Pat. No. 3,959,012)
It is considered that in such a cell, a reaction represented by Li+1/2PbI.sub.2 .fwdarw.LiI+1/2Pb takes place as a whole, and the open circuit voltage (O.C.V.) is about 1.8 V. LiI formed by the reaction has lithium ionic conductivity to some extent and accordingly, the supply (to the cathode-active material) of lithium ion (Li.sup.+) which transfers from the anode-active material (Li) via the solid electrolyte to the cathode-active material, continues without interruption, whereby the cell reaction proceeds as a whole.
However, in this case, PbI.sub.2 constituting the cathode-active material has no adequate electronic conductivity, and accordingly, there has been a disadvantage in the form of a degradation of the discharge characteristic (particularly, a drop of the electromotive force as time lapses, i.e. short useful life) due to, e.g., an increase of the internal resistance. In order to increase the electronic conductivity of the cathode-active material, it has been common to add to PbI.sub.2 an electronic conductor such as a powder of lead (Pb) or a powder of copper (Cu) or graphite which is nobler than Pb.
However, these electronic conductors have poor Li.sup.+ conductivity, and accordingly their addition tends to lead to a hindrance of the transfer of Li.sup.+ to the cathode-active material as a whole, whereby a smooth process of the cell reaction is hindered. Consequently, the cathode-active material sealed in the cell loses its function to receive Li.sup.+ without being wholly consumed. Thus, the consumption rate of the cathode-active material decreases. In other words, there will be a substantial amount of the cathode-active material left unreacted by the cell reaction. Further, the addition of these electronic conductors results in an increase of the weight or volume of the cathode-active material, which in turn results in an increase of the total weight or volume of the cell, whereby the energy density or the power density of the cell decreases. Such a result is undesirable.